The present invention relates generally to a drive roller unit for driving articles to be conveyed on a conveyor track.
Roller-type or ball-type conveyor tracks in which articles to be conveyed thereon are supported movably on a plurality of support rollers or balls include drive roller units for driving the articles to be conveyed, such as freight containers. A preferred area of use of such arrangements are freight loading systems in air freight traffic in which the containers are transported in the freight compartment of an aircraft on roller-type or ball-type conveyor tracks to move the containers to the anchorage locations at which they are to be disposed during a flight.
A drive roller unit as is disclosed for example in U.S. Pat. No. 3,698,539 comprises a base frame which in use is secured to the floor of a freight compartment and on which a mounting frame structure is pivotably mounted. In the vicinity of its pivot axis the mounting frame structure carries a motor while at a spacing relative to its pivot axis it carries a drive roller which is mounted rotatably but in a stationary position with respect to the mounting frame structure. The axis of rotation of the drive roller extends parallel to the axis of rotation of the motor. That unit also includes a cam control arrangement which is connected to the free end of the mounting frame structure and by means of which the mounting frame structure can be pivoted from a lowered rest position into a raised operative position, relative to the base frame of the unit. The output shaft of the motor is coupled by way of a transmission train to the input of a planetary transmission which forms a main transmission arrangement and which has two outputs of which one is coupled by way of a further transmission arrangement to the drive shaft of the drive roller while the other output of the planetary transmission serves to drive the cam control unit. For that purpose said other output of the planetary transmission has a tooth arrangement meshing with a gear which is carried non-rotatably on a shaft which is arranged on the mounting frame structure in the immediate vicinity of the pivot axis thereof. In order to transmit the torque from there to the cam control unit which is disposed at the opposite free end of the mounting frame structure, a chain wheel is non-rotatably mounted on the above-mentioned shaft which carries the gear, and the chain wheel is drivingly connected by way of a chain to a second chain wheel which is non-rotatably carried on the shaft of the cam control unit.
In the rest condition of the unit the mounting frame structure lies on the base or main frame so that the drive roller is in a lowered position at a spacing from the bottom of a freight container which is moved thereover on for example a roller conveyor track. In order to apply to the freight container a force for maintaining movement thereof, the motor is first switched on to drive the input of the planetary transmission. At that time the drive roller is prevented from rotating by virtue of the inclusion in the arrangement of a slipping clutch with a predetermined level of braking torque, so that the drive moment of the motor necessarily acts by way of the other output of the planetary transmission to drive the cam control arrangement which accordingly, by means of its cams which bear against the base frame, pivots the free end of the mounting frame structure upwardly into the operative position until the drive roller comes into engagement with the underside of the bottom of the freight container. Due to the weight of the freight container, that engagement between the drive roller and the bottom of the freight container causes the movement of the cam control unit to be blocked and the drive roller is pressed against the bottom of the freight container. That also causes the output of the planetary transmission, which drives the cam control unit, to be blocked, so that the other output of the planetary transmission, which drives the drive roller, now transmits to the drive roller a torque which overcomes the torque of the slipping clutch so that the drive roller, in a condition of being pressed against the bottom of the freight container, begins to rotate and the container is accordingly displaced on the conveyor track in a direction which is dependent on the direction of rotation of the drive motor.
A problem which arises with that arrangement in a practical context is that the entire transmission arrangement for transmitting torque from the motor to the drive roller on the one hand and the cam control unit on the other hand is of a very complicated configuration and involves a large number of components. That means that the costs involved in manufacture are high, while in addition the assembly is of a high weight and of large size, which is extremely undesirable in the context of aircraft engineering. In addition the large number of components which are movable relative to each other results in an undesirable susceptibility to wear and the periods of time between maintenance operations are short in order to ensure the high level of reliability required in operating aircraft.
Furthermore in that drive roller unit the slipping clutch is what is known as a fluid friction clutch, the operating characteristics of which are heavily dependent on temperature. Therefore, having regard to the high temperature differences which occur in operation in an aircraft freight compartment, it is necessary for the maximum torque of the slipping clutch to be selected at such a high value that the clutch torque does not fall below the minimum value required, even under the most disadvantageous temperature conditions. However that gives rise to very high force peaks when initiating the rotary movement of the drive roller when it is pressed against the bottom of a freight container, and such forces act on the floor structure of the aircraft freight compartment. That is extremely undesirable in practice as specific limit loadings in respect of the floor structure are not to be exceeded. In addition, shock loadings are basically considered to be disadvantageous.
More especially however the above-indicated prior drive roller unit is of a really large size and the individual components lie more or less unprotected next to each other so that in operation the unit is not only liable to suffer from fouling and defects and malfunctioning caused thereby, but it is also liable to suffer from damage due to objects which penetrate into the mechanism or by virtue of being directly subjected to an external force which interferes with the mechanism.